This invention relates to a method and apparatus for continuously coating one side only of a steel strip, running lengthwise, with a molten coating metal.
The practice to protect a base steel from corrosion by applying such metallic coatings as zinc, aluminum, Zn-Al and Pb-Sn alloys, etc. is widely known.
There are several applications that require the protective coating on one side only of the base steel. For example, one-side galvanized sheets are finding increasing use as a corrosion-resistant material for automative parts.
To meet such a demand, many one-side molten metal coating methods have been proposed. One known method, for example, achieves one-side coating by dipping base steel in a molten coating metal bath after forming a film of a material such as water glass on that side of steel which is to be left uncoated. But the forming of such a coating-preventive film makes the entire coating process complex. The film has to be removed on completion of metallic coating. The quality of the stripped surface may no longer be the same as that of the original bare steel. For example, phosphatability and solderability may be impaired.
According to this invention, a molten metal coating is given on one side only of a base steel without requiring the application of any pretreatment on the other side. Several similar pretreatment-less one-side coating methods are known, too. According to one known method, base steel is brought over a molten metal bath, with the side to be coated facing the bath surface so as to come in contact with a coating roll partly immersed in the metal bath. Another method lifts the surface of the metal bath by rotating an immersed impeller herein. The steel surface to be coated is brought into contact with this lifted or impelled portion of the bath. Japanese Official Gazette No. 49-25096 and Patent Publication No. 53-75124 disclose such one-side coating methods that bring the molten metal into contact with the steel surface by mechanical means. Immersed in the molten metal bath at high temperature, however, the coating roll and impeller employed by such known methods require special protection. Besides they cannot withstand use over a long period of time. Therefore, these methods are difficult to put into practice.
Japanese Patent Publication No. 53-60331 discloses practically the same one-side coating method. A pneumatic device produces an updraft to raise the metal bath surface for contact with the steel surface to be coated. This method too involves practical difficulties, especially in producing an updraft that is even and continuous widthwise.
A soldering device according to British Pat. No. 1,399,707 uses an electromagnetic pump for the transfer of molten metal. The electromanetic pump and an auxiliary pumps send forth molten metal through a rectangular passage, perpendicularly with respect to the metal bath surface. The resultant raised metal is supplied to or brought into contact with the bottom surface of base metal sheet. The molten metal is raised at the exit end of the rectangular passage enclosed by four guide plates, at entry and exit ends and on both sides, projecting above the metal bath surface. Excess metal returns on the bath from the entry and exit ends only, flowing over the guide plates on both sides.
In applying this method to the manufacture of one-side coated steel sheets, the raised metal must be uniformly supplied to or brought into contact with the steel surface to be coated, which can be attained by slightly pressing the steel sheet against the raised metal or supplying a slight excess of molten metal. Under such conditions, however, molten metal may be pressed against the side guide plates and sheet edges, so that there arises a high probability of molten metal flowing over to the surface that should not be coated.
As mentioned before, this method exerts a metal raising force perpendicular to the bath surface. When the balance between the rising force and the gravity-induced falling force of molten metal breaks, the raised metal tends to become wavy, preventing the uniform molten metal supply or contact.
This method controls the coating weight and widthwise coating distribution by no other means than the supply or contact of the raised molten metal. This calls for maintaining the line speed of the base steel as well as the quantity of molten metal for supply or contact at constant levels, which in turn require operating conditions as are too severe to be practical. In addition, the flowing of molten metal in the atmosphere greatly accelerates its oxidation. All these disadvantages make this method impracticable.
A method of one-side coating utilizing an electromagnetic pump for the transfer of molten metal is disclosed in Japanese Patent Publication No. 53-138930, based on an application filed by the inventors, and elsewhere. A steel strip whose surface has been pretreated ready for molten metal coating is introduced over a bath of molten coating metal kept in a non-oxidizing atmosphere. Coating metal is moved by electromagnetic induction so that part of the flowing metal rises to contact the bottom surface only of the steel strip. Subsequent wiper rolls or gas wiping controls the coating weight and smooths the coated surface.
All these publications have failed to provide any definite method or apparatus that insures stable production of one-side coated steels on a commercial scale. They lack considerations to the following important requistes to the stable commercial production of one-side coated steels;
(1) Effective application of an electromagnetic pump to the transfer of molten coating metal;
(2) Coating of one side only of steel strips of varying widths, leaving the opposite side uncoated, with molten coating metal, uniformly across the strip width including both edges;
(3) Protection of the non-coated surface from the contaminating by splashes and fumes of molten metal;
(4) Control of the coating weight and smoothening of the coated surface to the desired levels; and
(5) Provision of good surface quality and phosphatability to the non-coated side.